Sono-respond on thermosensitive polymer microgels based on cross-linked poly(N-isopropylacrylamide-co-acrylic acid)

Ultrasound (US) exposure strongly influenced thermosensitivity of microgels attracted with both N-isopropylacrylamide (NIPAM) and acrylic acid (AA) segments, due to that hydrogen bonds of carboxylic acid segments in microgels were broken by US and then the hydration with water occurred. US induced critical effects on the volume phase transition temperature of the swelled NIPAM gel (PNAM). It was observed after the US exposure that the particle size was increased and the phase transition of the microgels shifted toward larger temperature regions of the hydrodynamic diameter. FT-IR spectroscopic data of the swelled microgel showed that the free OH stretching band intensity of the COOH segments was enhanced by the exposure, but the band intensity returned to its original level without the US exposure. This meant that the US stimulus broke hydrogen bonding of the microgel and induced hydration of water in the hydrogel environment. Finally, regeneration of the hydrogen bonds in the microgel was occurred after the US exposure.     

Dual Responsive Poly(N‐vinylcaprolactam) Based Degradable Microgels for Drug Delivery

Poly(N‐vinylcaprolactam)‐based biodegradable microgels are prepared for drug delivery application via precipitation polymerization using diacetone acrylamide (DAAM) and dimethyl itaconate (IADME) as comonomers. The microgel particles are subsequently crosslinked by addition of adipic acid dihydrazide, which reacts with the ketone groups of DAAM. Itaconic acid (IA) groups are generated by the hydrolysis of IADME units inside the microgels resulting into both pH and temperature sensitive microgel particles. Volume phase transition temperature of the obtained microgels is influenced by both IA content and pH of the surrounding medium. Due to the incorporation of hydrazone linkages, the microgels show degradation under acidic conditions. These microgels can effectively encapsulate doxorubicin (DOX) as a model drug and show low DOX leakage under physiological conditions while rapid DOX release is observed at low pH. The results of the cytotoxicity assay further display that the DOX‐loaded microgels exhibit effective antitumor activity against HeLa cells demonstrating their great potential as drug delivery carriers for cancer therapy.     

Temperature-responsive poly(N-isopropylacrylamide) gels containing polymeric surfactants

Poly(N-isopropylacrylamide) (PNIPA) gel containing polymer surfactant poly(2-(methacryloyloxyl)decylphosphate) (PMDP) was synthesized and was found to show rapid volume phase transition above its transition temperature. Interestingly, the phase transition temperature of the PNIPA–PMDP gel was equal to that of the PNIPA gels alone. The concentration gradient of PMDP within the PNIPA gel can be obtained by applying an electric field on the gel, similar to the gel electrophoretic technique. The PMDP-gradient PNIPA gel clearly demonstrated the prevention of skin formation and the acceleration of the phase transition rate of the PNIPA gel by PMDP. The rapid volume phase transition allows potential applications of the PNPA–PMDP gel to soft actuators and drug delivery systems. Recently we also succeeded in synthesizing cylindrical microgels (0.8 μm in diameter, 5 μm in length) by a novel strategy where template-guiding synthesis and photochemical polymerization are combined. The obtained microgels can be characterized in individual level by a laser-trapping/Raman spectroscopy. In this article we also briefly described a famous gel system containing ionic surfactant capable of electrically driven actuators although it is not PNIPA gel.     

Temperature–Light Dual‐Responsive Au@PNIPAm Core‐Shell Microgel‐Based Optical Devices

Au nanoparticle (AuNP) core particles coated with a poly(N‐isopropylacrylamide) (pNIPAm) shell (Au@pNIPAm) are synthesized by seed mediated free radical polymerization. Subsequently, a temperature–light‐responsive photonic device is fabricated by sandwiching the Au@pNIPAm particles between two thin layers of Au. The optical device exhibits visual color and characteristic multipeak reflectance spectra, where peak position is primarily determined by the distance between two Au layers. Dual responsivities of the photonic device are achieved by combining the photothermal effect of AuNPs core (localized surface plasmon resonance (LSPR) effect) and the temperature responsivity of the pNIPAm shell. That is, the pNIPAm shell collapses as the temperature is increased above pNIPAm's lower critical solution temperature, either by direct heat input or heat generated by AuNPs' LSPR effect. To investigate the effect of AuNPs distribution in the microgels on the devices' photothermal responsivity, the Au@pNIPAm microgel‐based etalon devices are compared with that fabricated by AuNP‐doped pNIPAm‐based microgels; in terms of response kinetics and optical spectrum homogeneity. The uniform Au@pNIPAm microgel‐based devices show a fast response and exhibit a comparatively homogeneous spectrum over the whole slide. These materials can potentially find use in drug delivery systems, active optics, and soft robotics.     

Interplay between the glass and the gel transition

By changing the control parameters, many physical systems reach a slow dynamics regime followed by an arrested or a quasiarrested state. Examples, among others, are gels and glasses. In this paper, we discuss some experimental and theoretical results in polymer and colloidal systems, where gel and glass transitions interfere, and use models from Mode Coupling Theory(MCT)to illustrate the rich phenomenology observed. The continuous and the discontinuous transition lines, found in the MCT models,are considered suitable to describe respectively the gel and the glass transitions, so we suggest that the interplay between gel and glass may be interpreted in terms of the F_(13) MCT model, clarifying also the origin of logarithmic decays often observed in such systems. In particular, the theoretical predictions of the MCT in the F_(13) model are compared with Molecular Dynamics simulations in model systems for chemical gels and charged colloids.     

Jamming transition and new percolation universality classes in particulate systems with attraction

We numerically study the jamming transition in particulate systems with attraction by investigating their mechanical response at zero temperature (T=0). We find three regimes of mechanical behavior separated by two critical transitions--connectivity and rigidity percolation. The transitions belong to different universality classes than their lattice counterparts, due to force balance constraints. We also find that these transitions are unchanged at low temperatures and resemble gelation transitions in experiments on colloidal and silica gels.     

Microgels and fractal structures at interfaces and surfaces

The behavior of microgels near surfaces and their adsorption is studied by simple scaling theory. Two different types of microgels can be studied, i.e., fractal type microgels and randomly crosslinked polymer chains. In the first case the gel can be described mainly by introducing a spectral dimension. The second type requires more attention and uses the number of crosslinks as parameter. The main result is that soft gels with weakly coupled crosslinks and a low number of crosslinks adsorb much better than hard gels, with many crosslinks. Similar results for fractal gels and branched polymer are presented. Fractal gels with low connectivity adsorb easier than gels with a large connectivity dimension. We discuss also consequences on surface protection by microgels. Received: 11 August 1997 / Received in final form: 20 November 1997 / Accepted 22 January 1998     

Gels and glasses in a single system: evidence for an intricate free-energy landscape of glassy materials

In the free-energy landscape picture of glassy systems, their slow dynamics is due to a complicated free-energy landscape with many local minima. We show that for a colloidal glassy material multiple paths can be taken through the free-energy landscape. The evolution of the nonergodicity parameter shows two distinct master curves that we identify as gels and glasses. We show that for a range of colloid concentrations, the transition to nonergodicity can occur in either direction (gel or glass), accompanied by "hesitations" between the two. Thus, colloidal gels and glasses are merely global free-energy minima in the same free-energy landscape, and the paths leading to these minima can be complicated.     

Phase behavior of ionic microgels

We employ effective interaction potentials between spherical polyelectrolyte microgels in order to investigate theoretically the structure, thermodynamics, and phase behavior of ionic microgel solutions. Combining a genetic algorithm with accurate free energy calculations we are able to perform an unrestricted search of candidate crystal structures. Hexagonal, body-centered orthogonal, and trigonal crystals are found to be stable at high concentrations and charges of the microgels, accompanied by reentrant melting behavior and fluid-fcc-bcc transitions below the overlap concentration.     

有限尺寸胶体体系的二维结晶

从工业上的大尺寸晶体生长到实验室中受限小体系的结晶,结晶是普遍存在的物理现象,也一直是物理学中的重要研究课题.与大尺度结晶相变的研究相比,对于有限小尺度体系结晶过程的研究相对较少.本文通过设计具有吸引相互作用的胶体体系,在实验上研究了有限小尺寸胶体体系的二维结晶相变.通过计算和分析径向分布函数、泰森多边形以及取向序参量,发现有限小尺寸体系的结晶过程是从中央高密度区域开始,随着结晶的进行,周围液相减小而晶相增加,最后完全转变为晶态的过程.体系结晶速率呈现两个阶段:在结晶初期中央区域是高密度的亚稳态液体,会降低结晶自由能能垒,使得体系快速结晶;随后晶相长大,亚稳态液体消失,体系结晶速率变慢.此外,通过统计有序度参量的分布发现:在结晶过程中,序参量出现双峰分布,分别对应液相和晶相,与大尺度胶体体系的二维结晶行为一致,说明序参量分布的变化规律是二维结晶相变的重要特征.     

Interactions between colloids induced by a soft cross-linked polymer substrate

Using videomicroscopy imaging, we demonstrate the existence of a short-ranged equilibrium attraction between heavy silica colloids diffusing on soft surfaces of cross-linked polymer gels. The intercolloid potential can be tuned by changing the gel stiffness or by coating the colloids with a polymer layer. On sufficiently soft substrates, the interaction induced by the polymer matrix leads to large-scale colloidal aggregation. We correlate the in-plane interaction with a colloid-surface attraction.     

Rheology of sheared monodisperse granular suspensions

Sheared granular suspensions can either flow or be jammed. They show as well a ‘melting’ transition: partially ordered flowing states are found which can be melted into fully disordered arrangements of grains by sufficient shear. While these are well documented phenomena, the underlying mechanisms and their control parameters are still far from clear. Via Molecular Dynamics simulations, we study the rheology of a model system of sheared frictional monodisperse granular materials [7, 8]. In particular, we aim to understand the nature of a critical line separating crystallised and melted states and the “jammed” region in the phase diagram. We outline as well connections and differences with thermal glass formers and colloidal suspensions.     

Confined dynamics, forms and transitions in colloidal systems: from clay to DNA

Colloidal suspensions are a classic example of confining systems developing large specific surfaces, presenting a rich variety of shapes and exhibiting complex organization on a length scale ranging from 1 nm to several micrometers. Two distinct confined dynamics are generally considered in such systems: (1) the embedded fluid dynamics entrapped in the pore network with two main contributions, surface interaction and long-range connectivity, and (2) the dynamics of the host matrix, associated with a time evolution of the interfacial geometry. This last contribution is particularly important during dynamic and structural transitions of colloidal suspensions such as jamming, glass transition, phase separations and flocculation. It is generally believed that the characteristic time scale needed to describe colloidal movement and interfacial geometrical reorganization is much slower than the dynamics of the embedded fluid (except in the trivial situation where the fluid molecule is irreversibly adsorbed to a colloidal surface). Thus, few connections are made between these two distinct dynamics. In this presentation, we show how the slow and confined water dynamics at proximity of a colloidal surface provides an original way to probe colloidal shape and colloidal orientation dynamics. Two topics are presented. First of all, water field-cycling NMR relaxometry is used to probe the glass transition and the strong rotational slowing down of a colloidal system made of plate-like particles, a synthetic clay (laponite). Second, we analyze the case of long colloidal thin rods (either mineral or biologic such as DNA cylinders) dispersed in very diluted suspensions. At large distance and/or long time, these particles appear as a portion of a line. We discuss how the embedded fluid dynamics can be sensitive to this morphological crossover and may provide information about the particle shape. Some comparisons with recent experiments are presented.     

Phase transition in polymer gels due to local heating by illumination of light

The volume phase transition in gels induced by visible light and its related properties are presented, the mechanism of which is based on local heating of a polymer network by illumination of light. The gels consist of a covalently cross-linked copolymer network of thermosensitive N-isopropylacrylamide and a chromophore. Without light illumination, the gel volume changes sharply, but continuously at approximately 34°C when the temperature is varied. At a fixed temperature of an appropriate value, a discontinuous volume transition is observed when the light intensity is gradually changed. The phase transitions can be understood in terms of the temperature increment at the immediate vicinity of polymer chains due to the local heating via light absorption and subsequent thermal dissipation of light by the chromophore. The results can be qualitatively described by the Flory-Huggins mean-field equation of state of gels. In order to make clear the mechanism of the light-induced phase transition in the present system, we measured the light transmitting properties and the swelling as well as shrinking kinetics. These preliminary results are described semi-qualitatively by making use of a simple phenomenological model.     

Temperature-sensitive poly(N-Isopropyl-Acrylamide) microgel particles: A light scattering study

We present a light scattering study of aqueous suspensions of microgel particles consisting of poly(N-Isopropyl-Acrylamide) cross-linked gels. The solvent quality for the particles depends on temperature and thus allows tuning of the particle size. The particle synthesis parameters are chosen such that the resulting high surface charge of the particles prevents aggregation even in the maximally collapsed state. We present results on static and dynamic light scattering (SLS/DLS) for a highly diluted sample and for diffuse optical transmission on a more concentrated system. In the maximally collapsed state the scattering properties are well described by Mie theory for homogenous hard spheres. Upon swelling we find that a radially inhomogeneous density profile develops.     

Thermal Phase Transitions of IOTA Carrageenan in CaCl2 Solutions: A Fluorescence Study

The fluorescence technique was employed to study thermal phase transitions of iota (ι-) carrageenan (IC) in CaCl₂ solution. IC gels underwent coil to double helix (c-h) and double helix to dimer (h-d) transitions upon cooling. Upon heating IC gels presented dimer to double helix (d-h) and double helix to coil (h-c) transitions, showing hysteresis types of transition paths. Scattered light, I_sc and fluorescence intensity, I, were monitored against temperature to determine phase transitions. Transition temperatures were determined from the derivative of the transition paths. The critical gel fraction exponent, β, was measured and found to be in accord with the classic Flory–Stockmayer model.     

Electrically controllable metal–insulator-like transition in nanoparticle compacts

We report on the observations of tunneling transport in nanocompacts, where nanoparticles are packed into compact units using selective mass compositions and packing densities. An insulator-like thermal behavior in electron transport is seen in a very loosely packed 6-nm Ag nanocompact, whereas the densely packed 4.5-nm Au nanocompact displays a metal-like thermal behavior. Metal–insulator-like transitions, with the transition temperature can readily be tuned by controlling the bias voltage, are observed in the nanocompact consists of mixtures of 2.4 nm Ag and 4.8 nm core/shell Cu/Cu₂O nanoparticles. The resistivity across the metal–insulator transition is found to change by more than four orders-of-magnitude. At low bias voltages or small excitation currents, the metal–insulator transition occurs at ~190 K. The transition temperature can be tuned to reach the ambient temperature when a higher bias voltage or a larger current is allowed. Possible mechanisms that may produce the observed transport characteristics in the nanocompacts are discussed.     

Structure and flow of polyelectrolyte microgels: from suspensions to glasses

Polyelectrolyte microgels are soft particles due to their ability to de-swell osmotically. In dilute suspensions, they behave like soft Brownian particles. Above close-packing, they form pastes which share many features with glasses. The flow properties of suspensions and pastes are universal. They can be tuned at will to get the rheological behaviour which is desired. To cite this article: M. Cloitre et al., C. R. Physique 4 (2003).     

Mri measurements of T1, T2 and D for gels undergoing volume phase transition

Gels consist of crosslinked polymer network swollen in solvent. The network of flexible long-chain molecules traps the liquid medium they are immersed in. Some gels undergo abrupt volume change, a phase transition process, by swelling-shrinking in response to external stimuli changes in solvent composition, temperature, pH, electric field, etc. We report that during volume phase transition changes of NMR longitudinal relaxation time T(1), NMR transverse relaxation time T(2), and diffusion coefficient D of the PMMA gel, and D of the NIPA gel. We describe how the gels were synthesized and the reason of using the snapshot FLASH imaging sequence to measure T(1), T(2), and D. Since T(1), T(2) and D maps have identical field of view and data are extracted from identical areas from their respective maps, these values can be correlated quantitatively on a pixel-by-pixel basis. Thus a complete set of NMR parameters is measured in-situ: the gels are in their natural state, immersed in the liquid, during the phase transition. The results of spectroscopic method agree with that of snapshot FLASH imaging method. For the PMMA gel T(1), T(2) and D decrease when gels undergo volume phase transition between deuterated acetone concentration of 30% and 40%. At its contracted state, T(1) is reduced to a little less than one order of magnitude, T(2) over two orders of magnitude, and D over one order of magnitude, smaller from values of PMMA gel at the swollen state. At an elevated temperature of 54 degrees C the thermosensitive NIPA gel is at a contracted state, with its D reduced to almost one order of magnitude smaller from that of the swollen NIPA at room temperature.